The present invention relates to a vehicle steering control apparatus, and more particularly, to a steer-by-wire type steering control apparatus.
A first control apparatus used for steer-by-wire power steering is known in the prior art. In a vehicle that employs steer-by-wire power steering, the steering wheel is not mechanically connected to a steering wheel box, which is connected to the front wheels (steered wheels). The steer-by-wire type steering control apparatus drives the electric motor of the steering gear box in accordance with the steering angle of the steering wheel.
This conventional steering control apparatus has a back-up system so as to continue steering operation even when a breakdown occurs. Japanese Laid-Open Patent Publication No. 2002-37112 discloses a first prior art steering control apparatus provided with two control systems. One of the two control systems is a primary control system, and the other control system is a secondary control system. That is, the backup system of the first prior art steering control apparatus is formed by a redundant control system.
The primary control system includes a primary electric motor for driving a steering rod coupled to the steered wheels, a primary drive circuit for driving the primary motor, a primary control circuit for supplying control signals for driving the primary electric motor to the primary drive circuit, and various types of sensors. The structure of the secondary control system is identical to that of the primary control system. When both control systems are operating normally, the primary control system and the secondary control system operate the primary motor and the secondary motor to drive the steering rods in accordance with the steering angle of the steering wheel to avoid mutual interference between the primary motor and the secondary motor.
When the primary control system is impaired, the primary control circuit stops the primary electric motor, and the secondary control circuit drives the steering rod by means of the secondary steering motor.
Japanese Laid-Open Patent Publication No. 10-218000 (corresponding to U.S. Pat. No. 6,523,637) discloses a second conventional steering control apparatus for driving two steering motors. The control unit of this steering control apparatus distributes the drive forces of the two steering motors at a predetermined ratio.
The first and second conventional art steering control apparatuses control electric motors based on a detection value from a steering angle sensor for detecting the steering angle of a steering wheel. Thus, the torques of the two steering motors produce mutual interference.
Although the rotation angles of the output shafts of the electric motors detected by a rotation angle sensor provided to each electric motor is used for feedback control, the output shafts of the two electric motors are controlled at mutually different positions by the feedback control due to electric motor assembly errors and rotation angle sensor assembly errors. Therefore, the drive torque directions of the two steering motors are mismatched, and the synthesized torque is insufficient. Furthermore, noise and vibration are generated, and the electric motors are heated.
In the first conventional prior art apparatus, a detection mechanism is provided for detecting mutual torque interference of the steering motors of both control systems. When the mutual interference detection mechanism detects mutual interference, the operation of one of the control systems (including the operation of the electric motor) is stopped.
However, when mutual torque interference occurs, one control system is stopped and the steering rod coupled to the steered wheels are driven by the electric motor of the other control system, even though both control systems were operating normally. Therefore, this method is undesirable when both control systems are operating normally.
In the second conventional prior art apparatus, a primary steering motor and a secondary steering motor having mutually different configurations and performance are disposed at different locations. Since the performance (characteristics) of the two steering motors are mutually different, there is limited freedom for torque distribution to the two steering motors. Furthermore, a difference occurs in the steering operation sensitivity before and after impairment occurs regardless of which motor is impaired.